Autonomous Underwater Vehicles (“AUV”) and other devices that drive underwater are generally required to be very close to neutrally buoyant for proper underwater operation. As such, a vehicle that weighs about 42 pounds in air weighs about negative 7 ounces in water if the buoyancy is +1%. When operating an AUV there is a real fear that the AUV could become stuck underwater in mud or caught on a rope or line, and with so little upward force it is not likely that the AUV would be able to force itself back to the water surface for recovery.
If an AUV drives into mud nose first, it might take significant upward force (buoyancy) to pull the AUV free and send it to the water surface. This could require about 2-10 pounds of upward force to have the AUV pulled free from the mud. The normal operating model is to send divers down to free the AUV and let it float to the water surface. One method of creating extra buoyancy is to carry one or more ballast weights, e.g., lead weights, at neutral buoyancy, then drop a weight using an automatic release to increase the AUV's buoyancy. Emergency ballast weights have been used on many underwater vehicles, but such weights typically provide only a few extra pounds of buoyancy, or a few percent of the body weight.
Underwater devices such as AUVs present difficulties that prevent or restrict the adding of safety devices to the body of the vehicle. For example, their hulls or internal structures are usually pressure vessels to keep electronics contained therein dry while underwater at some pressure, thereby making access through the hull for installation or deployment problematic. Space within the underwater vehicle is very limited to add safety devices, but many underwater vehicles add some form of emergency device. The implementation of the emergency device is usually tightly integrated into the design of the underwater vehicle, burdening the underwater vehicle with the emergency device's cost and complexity. Conventional emergency devices include 1) a drop or ballast weight that can be dropped from the underwater vehicle automatically if it detects an emergency situation to add many pounds of upward force (buoyancy) to help the vehicle free itself and get to the surface for recovery; 2) an inflatable bag filled with a gas providing large amounts of positive (upward) buoyancy aiding the vehicle to get to the surface of the water, such as the expandable bag or sleeve described in U.S. Pat. No. 4,271,552 issued Jun. 9, 1981 titled “Torpedo Floatation Device,” hereby incorporated by reference in its entirety, but the complexity of inflatable bags dominates the underwater vehicle design, so they are not generally used; 3) an acoustic locator pinger that sends a loud sound out at a particular frequency or rate so the vehicle can be located from the surface with directional location devices; 4) a small float, optionally with a Global Positioning System (“GPS”) to transmit the location of the vehicle in distress via radio signals, dropped on a long line that is released, floats to the surface, and is used to find the underwater vehicle so it can be pulled up or retrieved by dive; and 5) lights to aid divers in close range location of the distressed vehicle. While certain of these conventional emergency devices have been used on underwater vehicles, they are either ineffective and/or have been integrated into the main vehicle, adding significantly to the size, complexity, and cost of the main vehicle. Furthermore, these systems need to be planned in the early stages of a design of the underwater vehicle.
Accordingly, there remains a need for effective, efficient rescue and recovery of underwater vehicles.